CL2602 – Chemical Engineer’s Toolbox

Laboratory manual – Distillation

Dr Abhishek Lahiri

CL2602 – Distillation laboratory manual 1

Introduction and background

Distillation is one of the most common techniques for liquid-liquid separation, and
considered to be a classic chemical engineering unit operation. Distillation works on
the basis of vapor-liquid equilibrium. The operation takes advantage of volatility
differences of the liquid components in a mixture at a given equilibrium temperature
and pressure condition. By controlling the heat input, components with lower boiling
points are strategically sent upwards as vapor, while the components with higher
boiling points drop downwards as liquid. This equilibrium “stage” repeats over the
length of the column so that the vapor-liquid separation occurs at multiple instances,
leading to a large concentration difference of highly volatile material between the top
and bottom of the column.

Distillation can be carried out both in batch process and continuous process. Usually,
in organic chemistry, purification of chemicals or liquid-liquid separation is performed
in batch process. However, in a chemical engineering distillation setting, often a
continuous process is performed due to the large desired production rate.

Distillation and rectification are thermal processes used to separate or purify liquid
mixtures whose constituents are wholly miscible with one another. Both processes
work by the basic operations of evaporating a liquid and then condensing the resulting
vapours.

When a liquid mixture AB is brought to the boil, wherein the substance A has the molar
fraction x1, vapour arises from it. But substance A now has a different molar fraction
y1 in the vapour than it does in the liquid mixture AB. If we enter the corresponding
values for the vapour for all possible mixture ratios of AB in a diagram, we obtain the
curve of equilibrium (see Figure 1). This curve indicates the mixture ratio of the vapour
given off by the boiling liquid.

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Figure 1 – Example of an equilibrium curve for a liquid mixture containing two
components A and B.
Curves of equilibrium are different for different substance mixtures. The more the
curve arcs away from the diagonal of the diagram, the greater is the increase in
concentration of substance A from liquid to vapour. The collected condensate is
therefore richer in low-boiling constituents compared with the initial mixture. The
proportion of high-boiling constituents in the residual mixture is correspondingly
higher. In distillation/rectification therefore, the vapour phase has a different mixture
ratio than the liquid phase. This fact is the basis of both processes.

In distillation, a liquid mixture is brought to the boil and the resulting vapour is drawn
off and condensed. This condensed liquid is known as the distillate. The remaining
mixture is the residue.

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Distillation apparatus consists of:

• an evaporator to heat the initial mixture and collect the high-boiling component
(residue).
• a condenser to condense the low-boiling component (distillate).
• one or more tanks to collect the distillate.

Distillation does not produce a complete separation of the initial mixture. It just divides
the initial mixture into two mixtures with different concentrations. There is also the
process in which the condensate is collected in different vessels, known as fractions,
as distillation time passes. This is known as fractional distillation.

Rectification

The result of the separation obtained by distillation is not sufficient in all cases. To
improve separation, the collected distillate should be distilled again. This process
should be repeated until the required material is satisfactorily enriched. Multiple-stage
distillation of this type is expensive and energy-intensive.

Consequently, rectification is applied (for a schematic view see Figure 2). In

rectification, the released vapour is first passed through a vertical tube called the
column before it reaches the condenser. Instead of all of the distillate collected in the
condenser being drawn off straight away, part of it is returned to the column as the so-
called reflux.

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Figure 2 – Schematic diagram of a rectification unit.
The difference between rectification and distillation is that the rising vapour mixture
and returning condensate flow counter to one another. The process is therefore also
known as reverse flow distillation.

The returned condensate now runs down the column and, at the next level down, is
forced into contact with the rising vapours on suitable internals for the purposes of
material exchange. The result is an exchange of material and heat between the rising
vapours and the falling liquid. The high-boiling component condenses first from the
rising vapours. The resulting condensation heat causes more low-boiling components
to evaporate in turn. The result is that the liquid flowing back to the bottom of the
column is enriched with high-boiling components. The vapours at the top of the column
are correspondingly richer in low-boiling components.

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Rectification can be understood as being multistage distillation, but with just one
evaporator and one condenser. This improves efficiency, because at the transition
from one stage to the next the vapour does not have to be first condensed and then
reheated and evaporated. Thus the number of stages in rectification is comparable to
the number of series-connected distillation stages.

At the top of the column, the distillate is divided into one part reflux and one part top
product. The ratio of reflux to top product is called the reflux ratio. It is an important
characteristic value for operation of the unit. The reflux ratio may assume values from
zero to infinity.

Aims

The main aim of the experiment is to understand and apply basic concepts in
distillation of binary mixtures. Continuous distillation of isopropanol- water mixture will
be investigated. Efficiency of the column will be estimated and effect of reflux ratio on
separation will be observed.

Learning outcomes from this laboratory are:

• Understanding of the principles involved in distillation processes.

• Ability to perform calculations related to the design and operation of distillation
processes.
• Appreciation of the safety considerations involved in the operation of distillation
processes.

Safety

Before conducting the experiment it is necessary that you have read the safety
assessment (available on Blackboard) as well as any other relevant safety
documentation and are aware of the hazards associated with the experimental work.
All experimental work should be conducted under the supervision of an authorised
user (i.e. the demonstrator or an instructor).

Particular hazards for this experiment include:

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 • The risk of burns from touching hot equipment or liquids. Heat resistant gloves
will be provided to users of the column, and care should be taken when
performing the experiment.
• Isopropanol is a flammable liquid. Hence it should be kept away from all
sources of ignition.

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Experimental procedure

Figures 3 and 4 show an overview of the experimental apparatus as well as a piping

and instrumentation diagram for the system.

Figure 3 – Overview of the experimental apparatus

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Figure 4 – P&ID of the experimental apparatus.

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The experimental procedure is as follows:

1. The evaporator will be filled with a mixture of isopropanol and water; this will be
approximately 20-30% isopropanol (by volume).
2. Take a sample from the evaporator tank to measure the density of the feed. To
do this:
a. Make sure you are wearing appropriate PPE. This includes your labcoat,
safety glasses, gloves and heat-proof gloves.
b. Open V-25.
c. Carefully open V-27. Collect the same using a large plastic measuring
jug. You only need to collect approximately 50-100 mL of sample.
d. Once the sample has been collected close V-27 and V-25.
e. If the sample is hot allow it to cool to room temperature before measuring
the density.

When taking a sample from the evaporator tank it is very important that you
take care as the liquid can be quite hot.

3. To measure the density:

a. Weigh an empty and dry graduated cylinder.
b. Fill this with a known volume of the sample.
c. Reweigh the graduated cylinder. Note down the masses and volume,
this can be used to determine the density of the sample.

Measured density values can be converted to the concentration of propanol by

looking up tables which give the density as a function of the mixture
composition. These can be found in the literature (see the reference list).
4. Press Reset on the differential pressure display
5. Unit operation by PC. Set and start the measurement data acquisition.
6. Set the reflux ratio to 100% initially. Open the condenser water supply and
adjust it to about 150 L/h
7. Switch on the heater. Set the heating percentage to 100%, this corresponds to
4000 W. When the evaporator temperature (T3) reaches 85 °C is reached,
reduce the heater to 30% of the maximum (1200 W).

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 8. Wait for distillate to appear at the phase separation tank (B3), reflux begins and
the system reaches a state of equilibrium (temperatures constant about 20
minutes after first appearance of distillate). Once the system is stable change
the reflux ratio to 50%.
9. When the minimum sample volume is reached (50 – 100 mL), withdraw the top
product from the top product tank (B4). Determine the density and temperature.
10. At the same time take a sample from the evaporator tank and measure the
density of the feed to the system.
11. Change the reflux ratio to 100%. Leave the system for 5 minutes.
12. Take a sample from tank B3. Determine the density and temperature.
13. Take a sample from the evaporator tank, determine the density and
temperature.
14. The demonstrator will turn the system off.

Report

Prepare a short report that clearly and concisely states the aim of the experiment, and
summarises key results. In this report please:

1) Describe the safety concerns and how to manage them in a distillation

process.

2) Determine the required theoretical minimum number of trays (please show

your calculations).

3) Discuss the effect of the reflux ratio in distillation columns and how changing
the reflux ratio will affect your results.

References:

1. P. A. Schweitzer, Handbook of separation techniques for chemical engineers,

McGraw Hill, 4th Edition

2. Advanced Distillation Technologies: Design, Control and Applications, John

Wiley and Sons, 1st Edition

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 3. Chu, K.-Y. and A.R. Thompson, Densities and Refractive Indices of Alcohol-
Water Solutions of n-Propyl, Isopropyl, and Methyl Alcohols. Journal of
Chemical & Engineering Data, 1962. 7(3): p. 358-360.
https://doi.org/10.1021/je60014a011

4. R. H. Perry, D. W. Green, Perry's Chemical Engineers' Handbook. (McGraw-

Hill, New York, 2008), . 9th Edition. See Chapter 2, Section 2-109

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